Re: [PATCH v8 10/46] KVM: guest_memfd: Wire up core private/shared attribute interfaces

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:31, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> With in-place conversion, guest_memfd is able to track the private/shared
> status of memory. Use a global flag to toggle between tracking
> private/shared status per-vm or within guest_memfd.
>
> When queried for supported vm memory attributes, return 0 if attributes are
> tracked in guest_memfd.
>
> When querying for memory attributes over a range, look up memory attributes
> based on the flag's state at query time.
>
> For per-GFN memory attribute queries, choosing an implementation (VM or
> guest_memfd lookup) at KVM load time.
>
> The flag is always false for now and will be made toggle-able after all
> in-place conversion features are added in subsequent patches.
>
> If/since the flag is false, if CONFIG_KVM_VM_MEMORY_ATTRIBUTES is also not
> selected, the per-GFN memory attribute query defaults to returning
> 0 (false/not private).
>
> Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

> ---
>  include/linux/kvm_host.h |  4 ++++
>  virt/kvm/guest_memfd.c   | 22 +++++++++++++++++++---
>  virt/kvm/kvm_main.c      | 12 +++++++++++-
>  3 files changed, 34 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 27687fb9d5201..acb552745b428 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2560,6 +2560,8 @@ static inline bool kvm_mem_range_is_private(struct kvm *kvm, gfn_t start,
>  #endif  /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
>
>  #ifdef kvm_arch_has_private_mem
> +extern bool gmem_in_place_conversion;
> +
>  typedef bool (kvm_mem_is_private_t)(struct kvm *kvm, gfn_t gfn);
>  DECLARE_STATIC_CALL(__kvm_mem_is_private, kvm_mem_is_private_t);
>
> @@ -2568,6 +2570,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>         return static_call(__kvm_mem_is_private)(kvm, gfn);
>  }
>  #else
> +#define gmem_in_place_conversion false
> +
>  static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>  {
>         return false;
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index bca912db5be6e..e0e544ef47d69 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -926,6 +926,24 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
>
>  #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
> +static bool kvm_gmem_range_is_private(struct file *file, pgoff_t index,
> +                                     size_t nr_pages, struct kvm *kvm, gfn_t gfn)
> +{
> +       struct maple_tree *mt = &GMEM_I(file_inode(file))->attributes;
> +       pgoff_t end = index + nr_pages - 1;
> +       void *entry;
> +
> +       if (!gmem_in_place_conversion)
> +               return kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + nr_pages,
> +                                                         KVM_MEMORY_ATTRIBUTE_PRIVATE,
> +                                                         KVM_MEMORY_ATTRIBUTE_PRIVATE);
> +
> +       mt_for_each(mt, entry, index, end) {
> +               if (xa_to_value(entry) != KVM_MEMORY_ATTRIBUTE_PRIVATE)
> +                       return false;
> +       }
> +       return true;
> +}
>
>  static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
>                                 struct file *file, gfn_t gfn, struct page *src_page,
> @@ -946,9 +964,7 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
>
>         folio_unlock(folio);
>
> -       if (!kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + 1,
> -                                               KVM_MEMORY_ATTRIBUTE_PRIVATE,
> -                                               KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
> +       if (!kvm_gmem_range_is_private(file, index, 1, kvm, gfn)) {
>                 ret = -EINVAL;
>                 goto out_put_folio;
>         }
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8b238e461b854..01761f6e25d25 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -101,6 +101,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
>  static bool __ro_after_init allow_unsafe_mappings;
>  module_param(allow_unsafe_mappings, bool, 0444);
>
> +#ifdef kvm_arch_has_private_mem
> +bool __ro_after_init gmem_in_place_conversion = false;
> +#endif
> +
>  /*
>   * Ordering of locks:
>   *
> @@ -2422,6 +2426,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
>  static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm)
>  {
>  #ifdef kvm_arch_has_private_mem
> +       if (gmem_in_place_conversion)
> +               return 0;
> +
>         if (!kvm || kvm_arch_has_private_mem(kvm))
>                 return KVM_MEMORY_ATTRIBUTE_PRIVATE;
>  #endif
> @@ -2633,8 +2640,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_mem_is_private);
>
>  static void kvm_init_memory_attributes(void)
>  {
> +       if (gmem_in_place_conversion)
> +               static_call_update(__kvm_mem_is_private, kvm_gmem_is_private);
>  #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
> -       static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
> +       else
> +               static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
>  #endif
>  }
>  #else
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>